1. Field of the Invention
The present invention relates to a plasma reactor for diamond synthesis.
2. Description of the Prior Art
Diamond film synthesized from gas phase has potential applications in various industrial fields and a strong demand for diamond films is expected in the future. However, extensive research and development activities have a history of only about five years and there are still many subjects of technical development to be completed for the industrial applications of diamond films.
The most important subject at the present stage is the development of deposition techniques of a diamond film on a large area of the substrate surface. For example, semiconductor industry, which is the most prospective field of diamond film application, requires techniques capable of depositing a diamond film on the surface of a several inch silicon wafer. Nevertheless, no technique meeting such a requirement has been proposed. Conventional diamond thin film forming techniques will be explained hereinafter.
(1) Conventional Technique I
The conventional technique I employs a hot filament for decomposing the reaction gas and heating the substrate. The principle of the conventional technique I is simple and is considered to have the highest possibility of industrial application.
FIG. 5 is a schematic view of an apparatus for the conventional technique I. It consists of a quartz tube 61, an electric furnace 62 enclosing the tube 61 to maintain the substrate temperature at an appropriate temperature (about 650.degree. C.), a substrate holder 63 disposed within the tube to hold a substrate 64 thereon, and a tungsten filament 65 disposed within the tube 61 and above the substrate 64 placed on the substrate holder 63 at a small distance (about 1 cm) from the substrate 64. The filament is electrically heated at about 2000.degree. C. to decompose the reaction gas and to heat the surface of the substrate 64 to a temperature in the range of 800.degree. to 1000.degree. C., which is the optimum temperature for diamond synthesis. The interior of the tube 61 is evacuated by a vacuum pump, not shown and then a reaction gas (usually CH.sub.4 +H.sub.2) of an appropriate CH.sub.4 concentration and a pressure of about 40 Torr is supplied into the tube 61 to make a diamond film grow spontaneously on the surface of the substrate 64.
(2) Conventional Technique II
The conventional technique II employs microwaves to produce a stable plasma.
FIG. 6 is a schematic view of an apparatus for the conventional technique II. The apparatus consists of microwaves generator 71 for a microwave of 2.45 GHz in frequency, a quartz tube 73, and a waveguide 72 for guiding the microwaves to the tube 73. A substrate 74 is disposed within the tube 73 at the intersection of the waveguide 72 and the tube 73. The tube 73 is evacuated by a vacuum pump, not shown, a reaction gas (usually CH.sub.4 +H.sub.2) is supplied into the tube 73, and then microwaves are irradiated by the microwave generator 71 to produce a plasma which encloses the substrate 74. Consequently, a diamond film grows spontaneouslly on the surface of the substrate 74. Ordinarily, the temperature of the substrate 74 is in the range of 800.degree. to 1000.degree. C., and the pressure of the gas within the tube 73 is in the range of 30 to 50 Torr.
(3) Conventional Technique III
The conventional technique III also employs microwaves for producing a plasma. FIG. 7 shows a reactor utilizing electron cyclotron resonance for the conventional technique III. The reactor comprises a microwave cavity 82, a vacuum chamber 84 connected to the cavity 82 by means of an opening 85, a waveguide 81 for guiding the microwaves into the cavity 82, a quartz window 83 provided at the junction of the waveguide 81 and the cavity 82, and electromagnets 87 enclosing the cavity 82 and the vacuum chamber 84 to create a magnetic field of an intensity meeting conditions for the electron cyclotron resonance within the cavity 82. A substrate 86 is usually placed within the vacuum chamber 84. The vacuum chamber 84 is connected to a vacuum pump, not shown, to evacuate the cavity 82 and the vacuum chamber 84. The energy of the microwaves introduced into the microwave cavity 82 is efficiently consumed by the electron cyclotron resonance within the cavity 82 to produce a plasma of a reaction gas (usually CH.sub.4 +H.sub.2) supplied into the cavity 82. The plasma flows out into the vacuum chamber 84, and thereby a diamond film is formed on the surface of the substrate 86. The pressure of the reaction gas within the reaction chambers 82 and 84 is on the order of 1 Torr, and the intensity of the magnetic field is 850 gauss.
When the conventional technique I is applied for a large area deposition, it needs a delicate operation to control the substrate temperature properly. Moreover, it is likely that the filament is elongated during the diamond deposition. This causes a change in the distance between the filament and the substrate during the reaction and hence it is not possible to obtain a diamond film of a desired quality.
The conventional technique II using 2.45 GHz microwaves of about 12 cm in wavelength is subject to restriction on the size of the quartz tube to avoid the leakage of the microwaves. To avoid the microwave leakage, the diameter of the tube must be less than 6 cm. Therefore, the size of the substrate is limited to less than about 1 cm.times.1 cm to ensure that the substrate is immersed in the plasma and the wall of the tube is not heated by the plasma to an excessively high temperature.
The conventional technique III usually uses the gas pressure of less than a few Torr and is able to produce a large volume plasma by applying microwaves to the reaction gas. However, it is not possible to synthesize diamond at a reasonably high growth rate under such a low gas pressure: the gas pressure must be increased at least to a range of 30 Torr or greater and accordingly the power of the microwaves must also be increased to about 1 kW or above. Under such conditions, the quartz window 83 and the cavity 82 are heated to an excessively high temperature and are etched by the reaction gas. In the worst case, the quartz window 83 and the walls of the cavity 82 are damaged.
To solve the foregoing problems in the conventional techniques, the inventors of the present invention have previously proposed a plasma reactor for diamond synthesis as shown in FIG. 8 in Japanese Patent Application No. 62-245068.
Shown in FIG. 8 is a microwave generator 91, a waveguide 92, an antenna 93, a microwave window 94 formed of quartz in a dome shape and disposed directly above the antenna 93, a vacuum chamber 95 for a generation of a reaction gas plasma, a substrate holder provided within the vacuum chamber 95, a substrate 97 mounted on the substrate holder 96, a plasma 98 surrounding the substrate 97, a reaction gas inlet port 99 attached on the wall of the vacuum chamber 95, and a reaction gas outlet port 100 attached on the wall of the vacuum chamber 95. Note that the antenna 93 is mounted on a quartz plate 101.
This plasma reactor for diamond synthesis has no external heating device. Microwaves generated by the microwave generator 91 are guided by the waveguide 92 toward the antenna 93, the antenna 93 changes the direction of propagation so that the microwaves propagate through the microwave window 94 into the vacuum chamber 95. This plasma reactor for diamond synthesis is capable of forming a diamond thin film on the substrate 97, such as a silicon wafer, of 3 in. in diameter.
The plasma is uniformly produced around the microwave window 94 because the microwave window 94 has a dome shape and hence the local heating of the microwave window 94 to an excessively high temperature rarely occurs. However, since the microwave window 94 is exposed directly to the plasma 98, the microwave window 94 is slowly etched by the plasma when the gas pressure is 30 Torr or higher and the microwave power is 2 kW or higher. Consequently, Si and SiO.sub.2, produced by etching the microwave window 94 formed of quartz are incorporated into the diamond film and, moreover it is possible that the microwave window 94 is broken by excessive heating. Accordingly, the gas pressure is limited to 30 Torr below and the power of the microwaves is limited to 2 kW or below. Therefore, the growth rate of the diamond film is only about 0.3 .mu.m/hr at the maximum. The gas pressure and the power of the microwaves must be increased to obtain a better growth rate.